Investigations of ternary phase relations in a CeO₂-Gd₂O₃-ThO₂ system

Abstract

The phase relations in a CeO₂-Gd₂O₃-ThO₂ system have been established, for the first time, after slowly cooling the samples from 1400°C. Ceria has been used as a surrogate material in place of plutonia. About 70 compositions in Th_1−xGdxO_2−x/2, Ce_1−xGdxO_2−x/2, (Th_0.5Ce_0.5)_1−xGdxO_2−x/2, (Ce_0.5Gd_0.5)_1−xThxO_1.75+x/4, and (Th_0.5Gd_0.5)_1−xCexO_1.75+x/4 systems were prepared. X-ray diffraction data revealed the presence of different phase regions, namely cubic fluorite-type solid solution, C-type solid solution, and biphasic regions containing C- and F-type solid solutions, and two F-type solid solutions could be delineated. This ternary system showed the existence of a wide cubic phase field. ThO₂ was found to dissolve up to about 40 mol% GdO_1.5. The GdxTh_1−xO_2−x/2 system has a biphasic region consisting of C- and F-type solid solutions, whereas GdxCe_1−xO_2−x/2 system supported only a single phase (F or C) throughout. A striking observation was that the anion-excess gadolinia, i.e., Gd_1−xCe_xO_1.5+x/2, was found to retain the C-type lattice, even after heat treatment, unlike pure gadolinia.